A comprehensive, accurate, and revisable alpha taxonomy is crucial for biodiversity studies, but is challenging when data from reference specimens are difficult to collect or observe. However, recent technological advances can overcome some of these challenges. To illustrate this, we used modern approaches to tackle a centuries-old taxonomic enigma presented by Fraser's Clawed Frog, Xenopus fraseri, including whether X. fraseri is different from other species, and if so, where it is situated geographically and phylogenetically. To facilitate these inferences, we used high-resolution techniques to examine morphological variation, and we generated and analyzed complete mitochondrial genome sequences from all Xenopus species, including >150-year-old type specimens. Our results demonstrate that X. fraseri is indeed distinct from other species, firmly place this species within a phylogenetic context, and identify its minimal geographic distribution in northern Ghana and northern Cameroon. These data also permit novel phylogenetic resolution into this intensively studied and biomedically important group. Xenopus fraseri was formerly thought to be a rainforest endemic placed alongside species in the amieti species group; in fact this species occurs in arid habitat on the borderlands of the Sahel, and is the smallest member of the muelleri species group. This study illustrates that the taxonomic enigma of Fraser's frog was a combined consequence of sparse collection records, interspecies conservation and intraspecific polymorphism in external anatomy, and type specimens with unusual morphology.

Department of Biological Sciences University of Texas at El Paso El Paso United States of America

Department of Biology McMaster University Hamilton ON Canada

Department of Life Sciences The Natural History Museum London United Kingdom

Department of Zoology Beaty Biodiversity Research Centre University of British Columbia Vancouver British Columbia Canada

Department of Zoology National Museum Prague Czech Republic

Florida Museum of Natural History University of Florida Gainesville FL United States of America

Forestry Research Institute of Ghana Kumasi Ghana

Institute of Vertebrate Biology of the Czech Academy of Sciences Czech Republic

Max Planck Institute for Evolutionary Anthropology Deutscher Platz Leipzig Germany

School of Biological Sciences University of Bristol Bristol United Kingdom

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Sex chromosome degeneration, turnover, and sex-biased expression of sex-linked transcripts in African clawed frogs (Xenopus)

A frog with three sex chromosomes that co-mingle together in nature: Xenopus tropicalis has a degenerate W and a Y that evolved from a Z chromosome